The present invention relates to a method and apparatus for the fine adjustment of a focused laser beam, in particular for the calibration of the beam deflection geometry in a laser machining apparatus.
The setting and fine adjustment of the beam deflection geometry in laser machining is a relatively laborious and expensive operation. For this purpose, a test pattern is usually cut into a workpiece with a laser beam, the pattern measured, the parameter setting corrected correspondingly and the entire procedure repeated, possibly several times, until the test pattern cut is in sufficiently accurate agreement with the ideal pattern.
In contrast, to adjust the deflection geometry in electronic scanning systems, among others, the following procedure is carried out: placing a defined calibrating pattern into the image plane, "scanning" the individual points of this pattern, the coordinates of which are known, with the projected scanning beam and correlating the necessary deflection data of the beam with the relevant coordinates. This method makes possible the automatic calibration of the system.
Such automatic calibration or fine adjustment procedure would be advantageous and desirable for laser processing machines, but has not been developed heretofore for the obvious reason that in the case of laser machines the material being worked is in the image or focal plane and consequently no calibration pattern can be applied to it. Furthermore, any appreciable reduction or attenuation of the laser intensity during calibration is not appropriate, as the beam would then no longer be representative of the subsequent actual operating conditions. A reduced beam intensity may alter the beam parameters and errors in the deflection geometry may be introduced by thermal effects. It is therefore important to be able to perform the calibration under conditions approximating the actual operating conditions as nearly as possible.
Conventional methods, which use cracks, holes, cutting blades or the like as test pattern components are entirely unsuitable because of the unavoidable working of the material under the full beam intensity.
The following patents are representative of the state of the art: U.S. Pat. No. 4,539,481 (EP-A No. 080 651), U.S. Pat. No. 3,485,996 (FR-A No. 1 551 347) and German Pat. DE-A-No. 2,559,925.
U.S. Pat. No. 4,539,481 discloses a method for adjusting the reference system of a laser processing device. In the method, the continuous-wave laser beam passes over the edge of the workpiece during each searching motion and the position of the workpiece is thereby determined relative to the reference system, which is then corrected accordingly. This method therefore positions the workpiece but does not correct nor adjust the beam geometry and parameters.
U.S. Pat. No. 3,485,996 discloses a laser welding apparatus, in which the workpiece is scanned by an auxiliary light source. Whenever the location on the workpiece of the weld to be carried out is detected by this scanning process, the laser beam is triggered.
German Pat. DE No. 2,559,925 discloses an apparatus for the centering adjustment of a laser beam on the optical axis of a microscope objective lens. A separate adjusting laser and a graticule that is centered with respect to the focusing optics are provided to center the laser beam on the optical axis. The deflection of the adjusting laser beam relative to the graticule is observed.